Why use this calculator
Changing tyre size affects far more than appearance. Overall rolling diameter determines what your speedometer reads, what your odometer records, how the anti-lock braking and stability control systems interpret wheel speed, and whether the tyre fouls the suspension or guard at full lock. A change of more than about three per cent in diameter is enough to put the speedometer outside legal tolerance in many jurisdictions and to confuse systems that were calibrated for the original fitment.
This calculator makes that comparison explicit before you buy, including the direction of the speedometer error, which matters because a larger tyre makes you travel faster than the dial shows rather than slower.
How to use it
- Read your current size off the tyre sidewall, where a marking like 225/45R17 gives the three numbers you need: 225 is the section width in millimetres, 45 is the aspect ratio as a percentage of that width, and 17 is the rim diameter in inches.
- Enter each into the current fields, then enter the size you are considering.
- If you are moving up a rim size, the usual approach is to drop the aspect ratio by five or ten points to keep the overall diameter close, which the comparison table will help you confirm.
- Finally, set an indicated speed so the calculator can show your true speed at that reading.
What each result means
- Diameter difference
- How much taller or shorter the new tyre stands, in millimetres and as a percentage. Three per cent is the usual tolerance for a change that keeps everything calibrated.
- True speed at the indicated speed
- How fast you are actually travelling when the speedometer shows your test speed. A larger tyre means you are going faster than the dial says.
- Current overall diameter
- The rolling diameter of the tyre fitted now, which every calibration in the car is based on.
- New overall diameter
- The rolling diameter of the proposed tyre. Getting this close to the original is the goal of any plus-sizing exercise.
- Current sidewall height
- The rubber between rim and road today, which absorbs impacts and determines how much the tyre flexes in corners.
- New sidewall height
- Sidewall on the proposed tyre. Less sidewall sharpens steering response but transmits more road shock and exposes the rim to kerb damage.
- Current revolutions per km
- How many times the wheel turns per kilometre now. This is the figure the speedometer and odometer are calibrated to.
- New revolutions per km
- Turns per kilometre with the new size. Fewer revolutions for the same distance is why a larger tyre makes the speedometer under-read.
- Ground clearance change
- How much the axle rises or falls, being half the diameter change. Check guard and suspension clearance at full lock before committing.
The formula
Sidewall height in millimetres = section width multiplied by the aspect ratio divided by 100
Overall diameter = rim diameter in inches multiplied by 25.4 to convert to millimetres, plus twice the sidewall height
Circumference = pi multiplied by the overall diameter
Revolutions per kilometre = one million divided by that circumference in millimetres
True speed = indicated speed multiplied by the new diameter divided by the old diameter, since the speedometer counts wheel revolutions calibrated to the original size
How this number is used in practice
Plus sizing is the practice of fitting a larger rim while reducing the aspect ratio so the overall diameter stays roughly constant. Each inch of rim adds 25.4 millimetres of diameter, so it has to be offset by about 12.7 millimetres less sidewall on each side, which is typically achieved by dropping the aspect ratio five or ten points and sometimes widening the tyre.
Done properly the speedometer stays accurate and the car looks and steers sharper. Done carelessly, by simply fitting the same aspect ratio on a bigger rim, the diameter grows by nearly four per cent in one step.
The direction of the speedometer error is what catches people out. A larger tyre covers more ground per revolution, and since the speedometer counts revolutions, it under-reads: you are travelling faster than it indicates, which is the dangerous direction for both speeding fines and stopping distances.
Most manufacturers deliberately calibrate speedometers to over-read slightly, typically by two to five per cent, because regulations in most countries allow a speedometer to read high but never low. A larger tyre eats into that built-in margin and can push the reading past accurate into genuinely optimistic.
Beyond the numbers, check three physical things before buying. First, clearance at full steering lock and full suspension compression, since a wider tyre can foul the strut or the inner guard only under those conditions. Second, load index and speed rating, which must meet or exceed the placard values on the door jamb regardless of how the size compares.
Third, your insurer and local regulations, because many jurisdictions limit how far fitment may deviate from the manufacturer's placard, and an undeclared modification can complicate a claim.
Worked examples
Moving from 225/45R17 to 245/40R18
Overall diameter rises 18.9 mm, or 2.98 per cent, just inside the 3 per cent tolerance
Fitting a taller 235/65R17 in place of a 225/45R17
Diameter jumps 16.24 per cent, so a shown 100 km/h is really 116.2 km/h
Going up a rim size without dropping the aspect ratio, 225/45R17 to 225/45R18
The extra inch of rim adds 25.4 mm, exactly 4.00 per cent, which is outside tolerance
When people use it
- Checking a proposed tyre or wheel upgrade before buying
- Working out the speedometer error introduced by a size change
- Plus sizing a wheel while keeping the rolling diameter close to standard
- Choosing a taller all-terrain tyre and checking the effect on gearing and clearance
- Verifying that a size offered as an equivalent really is close enough
Tips
- Stay within 3 per cent of the original rolling diameter. Beyond that the speedometer, odometer and stability systems all drift out of calibration.
- Each inch of extra rim adds 25.4 mm of diameter, so drop the aspect ratio by about 5 to 10 points to compensate.
- A larger tyre makes you travel faster than the speedometer shows, which is the dangerous direction of error.
- Match or exceed the load index and speed rating on the door placard, whatever size you choose.
- Check clearance at full steering lock and full suspension compression, not just standing still in the driveway.
- Keep all four tyres the same size. Mismatched rolling diameters damage differentials and confuse stability control.
What it assumes, and where it stops being accurate
- Uses nominal sidewall dimensions from the size marking. Real moulded diameters vary by a few millimetres between manufacturers for the same size.
- Assumes the speedometer is accurate with the original tyres, whereas most are deliberately calibrated to over-read by two to five per cent.
- Ignores tyre wear. A worn tyre has a measurably smaller rolling diameter than a new one of the same size.
- Ignores load-dependent deflection. Rolling radius under load is slightly less than half the free diameter.
- Does not check physical clearance, load index or speed rating, all of which must be verified against the vehicle placard.
- Does not address wheel offset or width suitability, which determine whether the tyre sits correctly and clears the suspension.
Terms used on this page
- Aspect ratio
- Sidewall height as a percentage of section width; the second number in a tyre size.
- Rolling diameter
- The overall outside diameter of the fitted tyre, which all vehicle calibrations depend on.
- Plus sizing
- Fitting a larger rim with a lower aspect ratio to keep overall diameter constant.
- Load index
- A coded number giving the maximum weight a tyre may carry at its rated pressure.
- Speed rating
- A letter giving the maximum speed a tyre is certified for under load.
Questions people ask
- What do the numbers on a tyre mean?
- In 225/45R17, the 225 is the section width in millimetres, the 45 is the aspect ratio meaning the sidewall is 45 per cent of the width, R indicates radial construction, and 17 is the rim diameter in inches. A load index and speed rating follow the size.
- How much can I change tyre size safely?
- Keep the overall rolling diameter within about three per cent of the original. That keeps the speedometer within legal tolerance and stops the anti-lock braking and stability systems from misreading wheel speed, which they calculate from the calibrated rolling diameter.
- Will a bigger tyre make my speedometer wrong?
- Yes. The speedometer counts wheel revolutions calibrated to the original diameter, so a larger tyre covers more ground per turn and the dial under-reads. You will be travelling faster than it shows, which is the direction that gets people fined.
- What is plus sizing?
- Fitting a larger diameter rim while reducing the aspect ratio so overall diameter stays about the same. It improves steering response and appearance while keeping the speedometer accurate. Each inch of rim needs roughly five to ten points less aspect ratio to compensate.
- Does a wider tyre always grip better?
- Not automatically. A wider tyre offers more contact area but can reduce pressure per unit area, hurt wet weather performance by aquaplaning sooner, increase fuel consumption and add steering weight. The compound and construction usually matter more than the width.
- Can I fit different sizes front and rear?
- Only if the manufacturer specifies a staggered fitment, which some performance and rear-wheel-drive cars do. Otherwise mismatched rolling diameters cause continuous differential wind-up on all-wheel-drive vehicles and confuse stability control on any car.
Use this with
Tyre Size Comparison Calculator answers one part of the question. These pick up where it stops: Fuel Efficiency Calculator, Car Running Cost Calculator, Speed Distance Time Calculator and Fuel Cost Calculator.
Where these figures come from
This calculation is jurisdiction-neutral arithmetic. No country-specific rules are applied.
- Tyre and Rim Association of Australia. Australian standards for tyre and rim fitment and dimensions
- US National Highway Traffic Safety Administration. Tyre safety, load ratings and labelling requirements
- European Tyre and Rim Technical Organisation. European dimensional standards for tyres and rims
Tyre Size Comparison Calculator produces an estimate for general information. It is not financial, tax, medical, legal or engineering advice. read the full disclaimer. Spotted a wrong number? Report it and it gets fixed. The form picks up this calculator and the inputs you used.